Process for producing core-plated electrical steel strip



United States 2909,4521- PROGESS'FOR PRODUCING CORE-PLATED ELEGTRICAL STEEL STRIP Richard A. Neish, Library, Pa., a'ssig'nqr to United States Steel Corporation, a corporation of New Jersey No Drawing. Application December 4, 1957 Serial No. 700,549 7 Claims. c1. 148 -62) conducting layer of inorganic material uponthe surface of the steel. A satisfactory core-plate must afiordhigh electrical resistivity at extreme thinness to provide asatis-' factory stacking or'space factor, must' be extremely ad'- herent to withstand the punchingand shearing incident to core manufacture, and be non-abrasive if e'c'onbinical die and bladelife is to be" achieved in such operations: In addition, it must be non-hygroscopic and inac'tive toward theoils used as coolants in-the finished tran form: ers. Prior to the present invention the most satisfactory core-plating agents have been-phosphate (phosphoric acid or ammonium phosphate) based solutions or slurri'es. While coatings made from aphosphate media are satisfactory in some respects,- the electrical resistivityis'seriiiis 1y decreased ('ofterr destroyed) when the core plate is stress-relief annealed in a reducing atm'o'sphere-L- This has" necessitatedthe useof less desirable oxidizing at'rr'io'spheres" in the stress-relief annealing of laminatio'n's. In addition; it is difii'cult to achieve an ecceptablespace'factor sirnul taneously with high electrical resistance and the processing must be closely controlled. Accordingly,- it is" the object of the present invention to provide a core plating" system free of the aforementioneddeficiencies partiw- 2,909,454 Patented Oct. 20, 1959 'ice i is preferentially oxidizing to carbon and silicon.

larly, acore plate that (1') retains satisfactory electrical resistance even-when stress r'eli ef annealed in hydrogen or dissociated ammonia,-and (2) has-a goodspace factor even when the coating has very high electrical resistance.

Modern practices in theproduction ofhi'gh grade elec' trical sheet comprise slabbing an ingot of suitable analysis, hot rolling the slab into strip which'is' then continuously pickled to remove hot mill-scale, cold reduced, continuously annealed, further cold reduced and again co1i tinuously annealed, following which the steel, in c'oil form, is box annealed at a temperature of 1900 to2300" F. in a reducing atmosphere, preferably hydrogen, to=devclop the desired grain-oriented structure having optimum mag netic properties. After box-annealing, thestrip is coreplated and coiled or sheared into" sheet. Fabricationinvolves shearing and punchingwhich necessitates subse quent stress-relief annealing. Because ofhigh"tempera-' tures used in the box-annealing,the steeliscoated 'witlra separating medium to prevent'adjacentwraps of th'e coilfrom welding together. The common practice in this regard is to roller-coat the strip aftenthe seco'nd continuous anneal With-a slurry ofmagne'sium' oxide-or ma nesium oxide and silica-flour, which is dried in situ' beforerecoiling. At" least one of" the" aforementioned continuousannealing steps isconducted in an atmosphere During box-annealing the silica resulting from this step reacts'with the separating coating to produce a thin adlie're'nt film, probably MgO-SiO adjacent the steel sur- {Ma a-overlain by a loose, powdery layer of unreacted I have found that when an aqueous chromic acid-magne'sium oxide mixture is applied over the adherent, glasslike layer of the separating coating and properly cured, the resliltanf coating can be successfully stress-relief ani healed in a strongly reducing atmosphere and is satisfactory in all other respects. This medium, I have found,

can contain 1.25 to about 3 pounds of chromic acid anhydr'ide crop per gallon of water together with 1 part by weight finely divided, U.S.P. magnesium oxide (MgO) for each 1.8 to 2.8' parts by weight of CrO The ratio of CrGg to M530 is particularly important in that when a ratio of 1 or less is used, the resulting coating is powdery and non-adherent; while a ratio of 4 or greater invariably results in hygroscopic coatings. Accordingly, the range 1.8 to 2.8 parts CrO to each part M'gO represents practical operating limits and inthe" commercial practice of my invention, it will be preferable to use a mixture of- 2.4 parts CrO to 1 part MgO.

The core-plating medium is best prepared by dissolving the chromic oxide in a portion of the water; mixing the MgO with' the balance of the water and then slowly adding the acid solution to the MgO slurry to form a yellowish colored aqueous solution, which may be clear or turbid depending uponthe" amounts of CrO and MgO used in its preparation. In general, CrO and MgO concentrations toward the upper end of the above specified ranges favor heavier core-plate coatings. Weight of coating,- however, is primarily controlled by metering the amount of solution appliedto strip. Coating weight between .005 and about .030 oz./sq. ft. of surface can be used. The space-factor loss with a coating of .015 iso'nly 0.4 to 0.5%.

' An extra light or active grade magnesium oxide, i.e., MgQ dried and calcined'at' relatively low temperature, facilitatespreparation of the solution since such material is more readily hydrated and dissolved. High fired Mg canbe used to replace part of the light-grade MgO if the particle size is sufficiently fine, but is not as satisfactory; 7 Other agents, such as acetic acid'commonly used in other chromate type film solutions, must be avoided in the present medium since I have found these cause blistering of core plate during curing, and bulking agents or extenders, such as the clay and silica-flour, used in conventional core-plating media adversely affect the stacking factor without beneficial eifect on surface resistivity.

The new core-plating medium can be applied to the steel byany of the conventional means but being a solution or a thin suspension rather than a slurry, is conveniently applied by immersion followed by squeegeeing through adjustable wiper-rolls to equalize and meter the coating. The coating must be cured by heating.

As in all core-plating operations curing is a function of both time and temperature. Since time in a continuous operation is a function of furnace length, heating characteristics and strip speed, and the temperature of a movin g'st ri p can not be determined accurately at present, curing practices'must be determined to a large extent the held. In the present instance, and within the limitations of temperature measurement, the mini-' The core-plating medium is applied directly to the annealed strip. The powdery layer of the MgO separating medium (SM coating) should be removed by brushing or scrubbing, the glass-like or adherent layer of this coating, however, must not be disturbed. This layer combines or co-operates with the above described chromic acid-magnesium oxide agents to produce a core plate of unusually high resistivity in combination with other desirable characteristics. Thus the steps of coating with MgO and heating to high temperature to form the thin glass-like film, probably MgO-SiO are essential steps in my core plating system.

The effect of variation in the weight of SM (MgO) coating as regards surface resistivity is illustrated by the following examples:

Table I Weight of Coating Franklin Test Values Test Panel (Amps. at

SM(MgO) (oz./sq. It.) 500 p.s.i.)

CrOa-MgO The Franklin Tests (A.S.T.M. A 34552 Interlarnination Resistance, Method No. 2) were made at a pressure of 500 p.s.i. after stress-relief annealing the test panels at 145 0 F. for 2% hours in a reducing atmosphere. The base metal was 14 gage, grain-oriented, T66 grade silicon strip; the SM coating medium, a slurry of 4.25 pounds of extra light MgO per gallon of water; the core-plating medium a solution of 1.25 pounds of CrO and 0.52 pound of MgO per gallon of water.

I have found that for the purposes of my invention, a minimum SM coating of about 0.006 oz./sq.ft. of surface must be applied. Coatings several times this amount can be used, however, a coating of about 0.015 oz./sq.ft. of surface provides an optimum balance of results, and is preferred. The SM coating medium must be an aqueous slurry of finely divided MgO. Any of the conventional MgO separating media may be used but I prefer a slurry of 4 to 5 pounds of extra light MgO per gallon of water, sufficient silica for the formation of the necessary silicate film being provided by surface oxidation of the silicon on the steel strip during the annealing steps. The slurry is applied in any conventional manner, e.g., by roller-coating, to the surfaces of the strip after the second continuous anneal. Excess water is removed by heating to about 650 F. and the desired silicate film is then formed during high temperature box-annealing.

As indicated in the above discussion, various modifica tions of practice are possible. Preferred practice, how: ever, contemplates processing the silicon steel in the regular manner through the second continuous anneal after which the strip is coated with a slurry of 4.25 pounds of extra light MgO per gallon of water, metered to provide a coating of about .015 oz./sq.ft. of surface. The coated strip is then passed through an oven heated to about 650 F. to dry and set the slurry, coiled and boxannealed at 1950 to 2300 F. for the usual time. After cooling the surfaces of the strip are wet-scrubbed, e.g., with rotary tampico brushes and rinsed to remove any powdery or unreacted MgO but leave intact the glassy film formed during the preceding high temperature treatment. The strip is then dried, e.g., by hot air blasts, and passed through a solution of 1.7 pounds CrO and 0.75 pound MgO per gallon of water, using squeegee rolls so regulated as to apply a coating of about 0.015 oz./ sq. ft. of surface and passing directly into a furnace or drying oven wherein the strip is heated to about 1400 F. to cure the coating to a spinel-like structure.

Typical results of the preferred practice of my invention as respects surface resistivity and space factor are Table II Franklin Test, Space Factor, Percent 1 Amps. at 250 p.s.i.-After Material Coating After Coating Before Coating Bef. Aft. Bet. Aft. BRA SEA 1 SRA SRA 1 BRA-The stress-relief annealing treatment was for 1 hr. at 1450 F. in dissociated ammonia.

2 Space Factor% is ideal.

The tabulated data are averages of tests from several coils of 0.014 gage, grain-oriented, grade 77 silicon steel strip. These coils after processing through the second continuous anneal were split and one half processed in accordance with the preferred practices of the present invention, and the other half by conventional practices. It will be noted that the resistivity of material B is decreased considerably by stress-relief annealing in a reducing atmosphere, whereas the electrical resistivity of the product of my method is not adversely affected. Further that the space factor of material A is significantly better than that of material B; in fact, core-plating by the present method improves the space factor as is shown by comparison of test results before and after coating.

While I have shown and described certain specific embodiments of my invention, it is obvious that certain modifications can be practiced-without departing from the scope of the appended claims.

I claim:

1. A method of producing core-plated electrical steel strip characterized by being stress-relief annealable in reducing atmospheres without loss of electrical properties including forming a silicon steel strip, treating said strip to produce a thin adherent film of magnesia and silica on the surfaces thereof, coating the treated strip with an aqueous solution of chromic oxide and magnesium oxide and then heating the strip to a temperature above 700 F. in a reducing atmosphere to cure said coating and produce a spinel-like core-plate having a high degree of electrical resistance combined with good space factor.

2. A method of producing core-plated electrical steel strip characterized by being stress-relief annealable in reducing atmospheres without loss of electrical properties including forming a silicon steel strip, treating said strip to produce a thin adherent film of magnesia and silica on the surfaces thereof, coating said strip with between 0.005 and 0.030 oz. of chromic acid anhydride and magnesium oxide per square foot of strip surface area by passing said strip through an aqueous solution consisting of 1.25 to 3 lbs/gal. of chromic acid anhydride and 1 part by weight finely divided magnesium oxide per each 1.8 to

2.8 parts by weight of chromic acid anhydride and then.

silica on the surfaces thereof, coating said strip withabout 0.015 oz. of chromic acid anhydride and magnesium oxide per square foot of strip surface area by passing said strip through an aqueous solution consisting of 1.7 lbs. of chromic acid anhydrideand 0.75 lb. of

i all:

finely divided, extra light magnesium oxide per gallon of water and then heating the strip to about 1400 F. to cure said coating and produce a spinel-like core-plate having a high degree of electrical resistance combined with good space factor.

4. A method of producing grain-oriented, core-plated electrical steel strip characterized by being stress-relief annealable in reducing atmospheres without loss of electrical properties including forming silicon steel strip, cold reducing said strip and annealing the cold reduced strip in an atmosphere which is preferentially oxidizing to silicon to provide silica at the surface thereof, coating said I strip with an aqueous slurry of magnesium oxide, drying said slurry and coiling said strip, box-annealing said coil at a temperature between 1900 and 2300 F. to develop the desired grain orientation and react said magnesium oxide and silica to produce a thin silicate film on the steel strip, coating said strip with an aqueous solution of chromic oxide and magnesium oxide and then heating the strip at a temperature above 700 F. to cure said coating and produce a spinel-like core-plate having a high degree of electrical resistance combined with good space factor.

5. A method of producing grain-oriented, core-plated electrical steel strip characterized by being stress-relief annealable in reducing atmospheres without loss of electrical properties including forming silicon steel strip, cold reducing said strip and annealing the cold reduced strip in an atmosphere which is preferentially oxidizing to silicon to provide silica at the surface thereof, coating said strip with at least 0.006 oz. of magnesium oxide per square foot of strip surface area by passing said strip through an aqueous slurry of magnesium oxide, drying said slurry and coiling said strip, box-annealing said coil at a temperature between 1900 and 2300" F. to develop the desired grain orientation and react said magnesium oxide and silica to produce a thin silicate film on the surfaces of the steel strip, coating said strip with between 0.005 and 0.030 oz. of magnesium oxide and chromic acid anhydride per square foot of strip surface, area by passing said strip through an aqueous solution consisting of 1.25 to 3 lbs/gal. of chromic acid anhydride and 1 part by weight finely divided magnesium oxide per each 1.8 to 2.8 parts by weight of chromic acid anhydride and then heating the said coated strip to a temperature above 700 F. to cure said coating and produce a spinel-like core-plate thereon having a high degree of electrical resistance combined with good space factor.

6. A method of producing grain-oriented, core-plated electrical steel strip characterized by being stress-relief annealable in reducing atmospheres Without loss of electrical properties including forming silicon steel strip, cold reducing said strip and annealing the cold reduced strip in an atmosphere which is preferentially oxidizing to silicon to provide silica at the surface thereof, coating said strip with at least 0.006 oz. of magnesium oxide per square foot of strip surface area by passing said strip through an aqueous'slurry of magnesium oxide, drying said slurry and coiling said strip, box-annealing said coil at a temperature between 1900 and 2300 F. to develop the desired grain orientation and react said magnesium oxide and silica to produce a thin silicate film on the surfaces of the steel strip, washing to remove substantially all unreacted magnesium oxide, coating said strip with about 0.015 oz. of magnesium oxide and chromic acid anhydride per square foot of strip surface area by passing said strip through an aqueous solution consisting of 1.7 lbs. of chromic acid anhydride and 0.75 lb. of finely divided extra light magnesium oxide per gallon of water and then heating the strip to about 1400 F. to cure said coating and produce a spinel-like core-plate having a high degree of electrical resistance combined with good space factor.

7. A method of producing core-plated electrical steel strip characterized by being stress-relief annealable in reducing atmospheres without loss of electrical properties including forming a silicon steel strip, annealing said strip to develop the electrical properties thereof, coating said strip with between 0.005 and 0.030 oz. of magnesium oxide and chromic acid anhydride per square foot of strip surface area by passing said strip through an aqueous solution consisting of 1.25 to 3 lbs./ gal. of chromic acid anhydride and 1 part by weight finely divided magnesium oxide per each 1.8 to 2.8 parts by weight of chromic acid anhydride and then heating the said coated strip to a temperature above 700 F. in a reducing atmosphere to cure said coating and produce a spinel-like core-plate thereon having a high degree of electrical resistance combined with good space factor.

References Cited in the file of this patent UNITED STATES PATENTS 2,472,592 Kiefer June 7, 1949 2,484,242 Nagel et al. Oct. 11, 1949 FOREIGN PATENTS 596,848 Great Britain Jan. 13, 1948 

1. A METHOD OF PRODUCING CORE-PLATED ELECTRICAL STEEL STRIP CHARACTERIZED BY BEING STRESS-RELIEF ANNEALABLE IN REDUCING ATMOSPHERES WITHOUT LOSS OF ELECTRICAL PROPERTIES INCLUDING FORMING A SILICON STEEL STRIP, TREATING SAID STRIP TO PRODUCE A THIN ADHERENT FILM OF MAGNESIA AND SILICA ON THE SURFACES THEREOF, COATING THE TREATED WITH AN AQUEOUS SOLUTION OF CHROMIC OXIDE AND MAGNESIUM OXIDE AND THEN HEATING THE STRIP TO A TEMPERATURE ABOVE 700*F. IN A REDUCING ATMOSPHERE TO CURE SAID COATING AND PRODUCE A SPINEL-LIKE CORE-PLATE HAVING A HIGH DEGREE OF ELECTRICAL RESISTANCE COMBINED WITH GOOD SPACE FACTOR. 